The present invention relates to a wiring substrate using a ceramic material as an insulating material thereof.
An example of prior art wiring substrates is disclosed on pages 344 to 349 of a paper entitled "LOW FIRING TEMPERATURE MULTILAYER GLASS-CERAMIC SUBSTRATE", by Y. Shimada et al published in the PROCEEDINGS 1983 33rd ELECTRONIC COMPONENTS CONFERENCE held by the IEEE and the ELECTRONIC INDUSTRIES ASSOCIATION. The substrate disclosed in the paper is manufactured by forming via-holes in a glass ceramic green sheet which constitutes an insulating layer, filling the via-holes with conductive paste, and sintering the green sheet and the paste filled in the via-holes. Although the conductive paste as taught by the paper is implemented with gold paste or silver-palladium alloy paste, the latter paste is more preferable than the gold paste from a cost standpoint. This paste contains silver powder whose particles are generally amorphous. When sintered, the paste containing the amorphous silver powder undergoes shrinkage by 25% to 30%, which is far greater than the shrinkage of a glass ceramic green sheet which is 12.5% to 13%. This brings about a problem that when paste containing such silver powder is used as a via-fill, cracks tend to be formed in the via-fill during sintering resulting in incomplete conduction. In addition, such paste has poor printing properties.